The present invention relates to a method and apparatus for writing servo tracks of a magnetic disk unit. In recent years, with an increasing demand for compact, large-capacity magnetic disk units (HDDs), magnetic disk units with disks having such a high information recording density as exceeding 100 megabits per square inch are now required.
To provide high-density disks, it is necessary to increase the number of tracks per inch in the disk radial direction (TPI: tracks per inch). In near future, to manufacture compact, large-capacity magnetic disk units as mentioned above, a track density more than 3,000 TPI will be needed. To attain the track density of 3,000 TPI, the pitch of tracks formed on the disk surface should be smaller than 10 .mu.m.
Therefore, more accurate head positioning is required in writing servo tracks onto the disk surface.
With reference to FIG. 11, a conventional servo track writing apparatus for a magnetic disk unit is described below. FIG. 11 is a plan view showing part of a servo track writing mechanism, in which a magnetic disk unit 2 as a subject of servo track writing is already placed on a stone surface plate 20.
First, components of the magnetic disk unit 2 is briefly described. A spindle motor 6 is mounted on a baseplate 4 of the magnetic unit 2, and a plurality of disks 8 are fixedly attached to a rotary shaft of the spindle motor 6. A rotary actuator 12 for moving a head 14 in the radial direction of the disks 8 is rotatably mounted with a pivot 10, which is fixed to the baseplate 4 outside the disks 8, as its rotary shaft. A voice coil motor (VCM) 16 for driving the rotary actuator 12 is disposed on the side of the pivot 10 opposite to the head 14. A protrusion 17 is formed on the VCM 16, and the tip of a linear positioner 18 for positioning the head 14 contacts with the protrusion 17.
The linear positioner 18 is a part of a servo track writing mechanism, and mounted on the stone surface plate 20 outside the magnetic disk unit 2 so that its tip contacts with the protrusion 17 of the VCM 16 of the magnetic disk unit 2. Driven by a driving device (not shown), the linear positioner 18 can move straight by a predetermined distance.
Next, a servo track writing method with the conventional servo track writing apparatus of FIG. 11 is briefly described. The linear positioner 18 is linearly moved by a predetermined distance and the tip of the linear positioner 18 pushes the protrusion 17 so that the head 14 moves by one track pitch. Pushed by the tip 17, the rotary actuator 12 rotates about the pivot 10 by a predetermined angle, so that the head 14 moves in the radial direction of the disks 8. During the above operation, a bias current is supplied to the VCM 16 to generate a balancing force in the direction opposite to the force exerted by the linear positioner 18, to ensure that the protrusion 17 is kept in contact with the tip of the linear positioner 18 (rigid-body-type contact).
As described above, in the conventional servo track writing, the head 14 for the servo track writing is positioned using the linear positioner 18 that links with the rotary actuator 12, which supports the head 14, through the protrusion 17.
However, the above conventional servo track writing method and apparatus is associated with various problems for the purpose of writing servo tracks of a magnetic disk unit that is required to have a high track density of more than 3,000 TPI.
As one problem, since the linear positioner 18 makes a linear movement in contrast to the rotational movement of the head 14 about the pivot 10, an error occurs in positioning the head 14 and is difficult to compensate. Another problem is that intermittent slips of a submicron order occur at the contact point of the linear positioner 18 and the protrusion 17 to increase the track pitch error.
Recently, the above problems have been solved by replacing the linear positioner 18 with a rotary positioner having a rotation axis that coincides with the rotation axis of the pivot 10. However, the following problems still arise even with the rotary positioner.
Even in the steady state after the head 14 has been positioned at a predetermined servo track writing position, a mechanical vibration at a resonance frequency that cannot be followed by the positioning servo occurs in the head 14 with an influence of the mass the linear positioner 18 (or rotary positioner) that is in rigid-body-type contact with the rotary actuator 12. For example, according to measurements with a magnetic disk unit used by the inventors and a conventional servo track writing apparatus, a vibration of 0.17 .mu.m in peak-to-peak amplitude at a frequency 560 Hz which cannot be followed by a servo system occurred in the steady state with an influence of the mass of a positioner.
In positioning the head 14, the forces of the linear positioner 18 and the VCM 16 are balanced with each other to provide the rigid-body-type coupling between the protrusion 17 and the linear positioner 18 (or rotary positioner). As a result, when the linear positioner 18 makes a step-function-type movement corresponding to one track pitch, a mechanical vibration occurs in the rotary actuator 12 as a transient response to such a step-like movement. That is, due to the large inertial mass of the linear positioner 18, the transient response to the step-like movement for the one-track seek of the head 14 does not attenuate in a short period, to make the settling time of the system longer. FIG. 12 shows a head settling time with the magnetic disk unit used by the inventors and the conventional servo track writing apparatus. As shown in FIG. 12, a settling time necessary to settle at a target position of 5 .mu.m is 12 milliseconds. If the head settling time is too long in a magnetic disk unit equipped with disks having a track density as high as 3,000 TPI, the throughput of the servo track writing step reduces by a non-negligible amount.
A further problem is that in the servo track writing in an actual manufacturing process the opening for the servo track writing to be formed in a top cover of a magnetic disk unit cannot be made large.